The invention relates to a method for handing off a mobile station from a first channel on a serving base station to a second channel on a target base station in a mobile communication system. The method also includes handing off the mobile station from the first channel to the second channel in the serving base station. The mobile station is transmitting at a first transmission phase on said first channel, said second channel being ready to take over the communication with said mobile station transmitting at a second transmission phase. The invention also relates to an apparatus for carrying this method into effect.
In a mobile communication system, e.g. a cellular mobile radio system, an inter-cell handoff is the actual switching of an ongoing call from a base station in one cell to a base station in another cell, while an intra-cell handoff is the actual switching of a connection from one channel to another channel in one and the same base station.
Inter-cell handoffs are performed in order to effect a change of base station, e.g. due to the mobile station moving in a cellular environment, or to relocate some traffic from a highly loaded base station to a less loaded base station.
An intra-cell handoff takes place e.g. if the quality on the currently used channel falls below the minimum accepted quality or during a call set-up procedure when handing off the mobile station from a control channel to a traffic channel.
When an inter-cell or intra-cell handoff is performed between unsynchronized channels or between channels having a phase difference unknown to the mobile station, the mobile station must resynchronize itself on to the new channel. This resynchronization process takes considerable time, e.g. 200 ms, resulting in a non-negligible interruption of the speech signal which can cause disturbing interruptions in the ongoing speech.
The patent application WO 9222966 shows a method in a cellular mobile radio telephone system for initially setting a time alignment parameter in a mobile station. The method comprises the steps of estimating an optimal time alignment parameter for use during communication on a new traffic channel of a desired base station, sending the estimated time alignment parameter to the mobile station, and setting the time alignment value in the mobile station.
The patent application EP 0430106 A2 shows a method for hand over in a mobile telephone system, by using mobile stations with two local oscillators. By having two local oscillators it is possible to receive on a first channel and at the same time transmitt on another channel. This can be used to reduce the time duration of making a hand over.